As is known and as is illustrated schematically in FIG. 1, an air-conditioning system 1, in particular of a vehicle 1a (illustrated schematically), comprises: an air-conditioning assembly 2; a cooling circuit 3 of the closed-loop type and a heating circuit 4, both connected to the air-conditioning assembly 2.
The cooling circuit 3 is provided with a duct 11 for supply of air to be treated, which has a first inlet 11a, communicating with the outside of the vehicle 1a, and a second inlet 11b, communicating with the passenger compartment (not illustrated) of the vehicle. The air-supply duct 11 has an outlet 11c, communicating with an inlet 2a of the air-conditioning assembly 2. A first selector 12 is set along the air-supply duct 11 for directing the flow at its inlets 11a, 11b towards the outlet 11c. In this ways the air to be treated can be taken in selectively from the environment external to the vehicle and/or from the passenger compartment of the vehicle (the so-called air-recirculation function) according to the position assumed by the first selector 12.
The air-conditioning assembly 2 comprises an evaporator 13 set at its inlet 2a and adapted to be traversed by the air coming from the air-supply duct 11.
The evaporator 13 is also traversed by a coolant, in particular a gas, for example R134a, which flows along a duct 14 connecting the elements that form the cooling circuit 3. During traversal of the evaporator 13, the air transfers heat to the coolant and cools off.
The coolant leaving the evaporator 13 is supplied, through the duct 14, to the inlet of a compressor 18, which is in turn connected at outlet to a condenser 19. Furthermore, a capillary 20 (or alternatively a thermostatic expansion valve) is set between the outlet of the condenser 19 and the inlet of the evaporator 13. The compressor 18 takes in, at a certain intake pressure, the coolant in the vapour phase from the evaporator 13 so as to obtain a control of the temperature of the air downstream of the evaporator 13, the condenser 19 receives the coolant in the vapour phase from the compressor 19, and the capillary 20 receives the coolant in the liquid phase from the condenser 19 to supply it in two phases (the vapour phase and the liquid phase) to the evaporator 13.
The air-conditioning assembly 2 further comprises a mixer 15 communicating through a duct 15c with an outlet of the evaporator 13. Set within the duct 15c is a fan 17, configured to create a forced flow of air from the evaporator 13 to the mixer 15.
The mixer 15 defines an internal chamber 24, defined within which are a first path 24c and a second path 24h separated from one another and selectable at inlet by means of a second selector 23, which supplies the air coming from the duct 15c to the paths 24h and 24c. In particular, the second selector 23 can be set in a first limit position (indicated by the dashed line), in which all the inlet air is supplied to the first path 24c, in a second limit position (not illustrated), in which all the inlet air is supplied to the second path 24h, and in a plurality of intermediate positions (one of which is indicated by a solid line), in which the inlet air is partialized between the two paths.
In particular, the second path 24h communicates with an outlet of the heating circuit 4, which is conveniently constituted by a heat exchanger of the liquid/air type, adapted to receive a flow of cooling liquid of the internal-combustion engine (not illustrated) of the vehicle 1a, in some cases through a control solenoid valve.
The chamber 24 also communicates at outlet with the passenger compartment through a diffuser 26, to which aeration mouths are connected.
In the mixer 15, the cold air coming from the evaporator 13, before being introduced into the passenger compartment of the vehicle by the diffuser 26 through the mouths, can be mixed with hot air coming from the heating circuit 4. In particular, the flow of cold air F1 at outlet from the fan 17 can be appropriately mixed with the flow of hot air F2 coming from the heating circuit 4 by means of the second selector 23. The second selector 23 can be positioned both so as to channel the entire flow of cold air F1 towards the diffuser 26 (so-called “all cold” position), without enabling any passage of cold air within the hot-air duct and thus preventing mixing of the hot and cold air, and so as to enable completely (“all hot” position) or just in part passage of the flow of cold air F1 within the hot-air duct, thus favouring mixing of the two flows of cold air F1 and hot air F2. The mixing can be controlled as a function, among other things, of a temperature that has been set (the so-called “set-point temperature”), designated in what follows by Tsp, required by the occupants of the vehicle and set via appropriate means for regulating the temperature inside the passenger compartment.
In particular, if we designate by Tin the temperature of the air at the inlet of the evaporator 13 (which can consequently be air coming from outside, or a mixture of air coming from outside and from the air-recirculation system), Tw the temperature of the cooling liquid at the inlet of the heating circuit 4, Tc the temperature of the air leaving the evaporator 13, Tt the temperature at the inlet of the diffuser 26, γ the fraction of flow of air in the hot-air duct, and ε the efficiency of the heating circuit 4, the following relation applies:TtTc=f(γ,ε,Tw,Tc)
As illustrated in FIG. 1, control of the air-conditioning system 1, and in particular control of the fan 17, of the compressor 16, and of the mixer 15, is obtained by means of an electronic control unit 28, receiving signals from various sensors present both inside and on the outside of the vehicle 1a (for example, internal-temperature and external-temperature sensors, humidity sensors, etc.).
In particular, in NP (Normal Production) systems, a fixed-displacement compressor is managed by the electronic control unit 28 also on the basis of the output of a temperature sensor, set downstream of the evaporator 13 and hence detecting the temperature Tc of the air leaving the evaporator. When the temperature Tc of the air leaving the evaporator 13 drops below a pre-set threshold (defined hereinafter as “disconnection threshold”) the compressor 18 is deactivated to prevent the water condensed on the surface of the evaporator from freezing and causing obstruction of part of the corresponding heat-exchange surface. The compressor 18 hence works in “on-off” mode with respect to the disconnection threshold, said threshold being set and fixed, for example, at a value of 3° C. Possibly, a hysteresis can be envisaged for reconnection of the compressor 18, which is actuated again when the temperature Tc exceeds a “connection threshold”, which has a value that is higher than the disconnection threshold and that is also pre-set and fixed, for example, at 5° C.
In a system of the type described above, a cooling capacity is normally produced that is excessive with respect to the one that would be necessary to guarantee thermal comfort conditions in the vehicle passenger compartment. This occurs certainly using fixed-displacement compressors, but also using a variable-displacement compressor of an internal-control type, both in conditions of low thermal load and in conditions of high thermal load, once the “cool-down” transient ends. In particular, a desired temperature is reached in the passenger compartment by mixing the flow of air at outlet from the evaporator 13 (which is in any case treated completely by the evaporator) with the flow of hot air at outlet from the heating circuit 4 (so-called “post-heating”). This enables raising of the temperature of the air introduced into the passenger compartment with respect to the temperature of the cooled air but clearly entails destruction of part of the generated cooling capacity, which, since the compressor is driven by the engine, in turn entails an increase in the energy used and in the consumption of the vehicle.